From 04bcd16187aabda85340d0b6e96d8ff0cd740974 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Tue, 25 Aug 2020 13:08:01 +0200
Subject: [PATCH] Reimplement commented-out code.

---
 .../Utils/CollectionChangedEventManager.cs          | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/src/Avalonia.Controls/Utils/CollectionChangedEventManager.cs b/src/Avalonia.Controls/Utils/CollectionChangedEventManager.cs
index fa3f684747..1e5ada8409 100644
--- a/src/Avalonia.Controls/Utils/CollectionChangedEventManager.cs
+++ b/src/Avalonia.Controls/Utils/CollectionChangedEventManager.cs
@@ -44,11 +44,14 @@ namespace Avalonia.Controls.Utils
                     this);
             }
 
-            //if (listeners.Contains(listener))
-            //{
-            //    throw new InvalidOperationException(
-            //        "Collection listener already added for this collection/listener combination.");
-            //}
+            foreach (var l in listeners)
+            {
+                if (l.TryGetTarget(out var target) && target == listener)
+                {
+                    throw new InvalidOperationException(
+                        "Collection listener already added for this collection/listener combination.");
+                }
+            }
 
             listeners.Add(new WeakReference<ICollectionChangedListener>(listener));
         }